Multi-user multiple-input and multiple-output (MU-MIMO) transmission is becoming a new system technique to enable high system capacity in both the upcoming IEEE 802.11ac and the LTE (long-term evolution) standards. As compared to single-user MIMO (SU-MIMO), MU-MIMO has several key advantages. First, MU-MIMO allows for a direct gain in multiple access system capacity proportional to the number of access point antennas. Second, MU-MIMO allows the higher degree spatial multiplexing gain to be obtained without the need for higher number of antennas at the mobile stations by keeping the intelligence and cost at the access point. Third, MU-MIMO appears immune to most propagation limitations plaguing SU-MIMO communications because multi-user diversity can be extracted even in a simple line of sight (LOS) propagation environment. As a result, the LOS propagation, which causes degradation in single user spatial multiplexing schemes, is no longer a problem in the multi-user setting.
While single-user MIMO (SU-MIMO) considers access to the multiple antennas that are physically connected to each individual terminal (e.g., user), multi-user MIMO (MU-MIMO) allows a terminal to transmit (or receive) signals to (or from) multiple users simultaneously. The typical MU-MIMO usage scenario in IEEE 802.11ac involves an access point (AP) or router first acquiring the MIMO channel state information (CSI) through channel sounding, computing and applying transmit beamforming (precoding) weights, and then simultaneously transmitting multiple spatial streams to more than one mobile stations (STAs). With proper transmit beamforming (precoding), partial spatial processing is done at the access point to separate the spatial streams among the multiple users, and the remaining spatial processing is done at the receivers to decode the multiple spatial streams received.
FIG. 1 (prior art) illustrates a MU-MIMO sounding and feedback process in a wireless system 100. Wireless system 100 comprises a transmitting access point AP101 and three receiving stations STA102-104 in one multi-user (MU) group. As illustrated in FIG. 1, each channel sounding and feedback process is followed by a series of MIMO frame exchange. For downlink transmission, AP101 (initiator or beamformer) first broadcasts a sounding announcement (e.g., null data packet announcement (NDPA) 111) to inform the intended stations (responders or beamformees) and a sounding packet (e.g., null data packet (NDP) 112) is then transmitted for the intended responders. Each beamformee estimates the channel during the preamble portion of the sounding packet. For uplink transmission, STA102 transmits feedback message 113 after receiving NDP 112 (with SIFS/RIFS), STA103 transmits feedback message 115 after receiving polling message 114, and STA104 transmits feedback message 117 after receiving polling message 116. The feedback messages contain the CSI (channel state information) and the average SNR (signal-to-noise ratio) to allow the beamformer to compute the transmit antenna (precoding) weights and to apply link adaptation for downlink (DL) MU-MIMO transmission.
Under the current IEEE 802.11ac sounding protocol, the sounding packet NDP is un-beamformed (e.g., not MU-MIMO pre-coded). Therefore, the channel information and SNR in compressed beamforming feedback report does not include inter-user or inter-STA interference or leakage among the multi-user group. The actual signal-to-interference-noise-ratio (SINR) (including the inter-user interference) during MU-MIMO transmission can be much smaller than the SNR feedback in the sounding and feedback process. Inter-user interference in MU-MIMO can be introduced by channel estimation errors, channel variations, or channel aging. Lack of knowledge for the inter-user interference and the interference source at AP side may lead to wrong modulation and coding scheme (MCS) selection for MU-MIMO transmission. This is because the interference power can be much larger than the noise power especially when SNR is high. For example, if the SNR at STA101 is 20 dB and the leaking interference power is 10% of the signal power, then the SINR became 10 dB, which may lead to three MCS level difference in making MCS selection. Lack of knowledge for the inter-user interference and interference source at AP side may also lead to unnecessary channel resounding and inaccurate MU group selection. Re-sounding overhead is significant because of channel feedback. A solution is sought.